The invention relates to a process for the ring chlorination of aromatic hydrocarbons in the presence of Friedel-Crafts catalysts and in the presence of co-catalysts in liquid phase.
The reaction of aromatic hydrocarbons, such as toluene, in liquid phase with chlorine gas to give ring-substituted chlorinated derivatives, such as monochlorotoluene, is known (Ullmanns Enzyklopadie der technischen Chemie (Ullmann's Encyclopaedia of Industrial Chemistry), 4th edition, Volume 9, page 499f.). In general this chlorination is carried out in the presence of Friedel-Crafts catalysts, such as iron (III) chloride, antimony chlorides or aluminum chloride. The chlorination product obtained is usually a mixture of isomeric monochlorinated and polychlorinated compounds. If FeCl.sub.3 is used, for example, a mixture of monochlorotoluenes and dichlorotoluenes is obtained from toluene; the main product of the monochlorotoluene fraction is o-chlorotoluene beside p-chlorotoluene and a small amount of m-chlorotoluene.
Since especially p-chloroalkylbenzenes, such as p-chlorotoluene, are useful intermediates, there has not been a lack of attempts in the past to control the chlorination in such a manner that the ratio of o- to p-chloroalkylbenzenes is lowered, that is, the attempt is made to find conditions which favour the formation of p-chloroalkylbenzenes.
It is known from U.S. Pat. No. 3,226,447 that by the addition of sulphur compounds having divalent sulphur to the Friedel-Crafts catalyst an o/p ratio of 1.2 can be obtained in the chlorination of toluene. The disadvantage of this process resides in the fact that this less than favourable ratio can only be achieved if antimony salts are used as Friedel-Crafts catalysts. Another disadvantage is that the required amounts of the catalyst components according to Example 16 of that application are very high, specifically 1% by weight for each of the two catalytic additives. As shown by the o/p ratio having a value of &gt;1, the result is still more o- than p-chlorotoluene.
DE-OS (German Published Specification) No. 1,543,020 and U.S. Pat. No. 4,031,144 also describe the chlorination of toluene, for example, with FeCl.sub.3 and S.sub.2 Cl.sub.2. The ratio obtained of o/p=1.03-1.10 is still unsatisfactorily high.
U.S. Pat. No. 4,031,147, U.S. Pat. No. 4,069,263, U.S. Pat. No. 4,069,264 and U.S. Pat. No. 4,250,122 describe the chlorination of toluene with Friedel-Crafts catalysts with the addition of thianthrenes or substituted thianthrenes. The most favourable o/p ratios obtainable are around 0.7, which, however, are only obtained either by using antimony salts or, if iron salts are used, only at very low reaction temperatures of about 0.degree. C. Both situations are extremely unfavourable for practical applications. Thus, the co-catalytic action of the thianthrenes in combination with the use of antimony salts is greatly impaired by traces of iron, which is difficult to realize in practice. In addition the reaction is strongly exothermic to such an extent that removal of the heat is about 0.degree. C. by ice/salt cooling becomes very expensive. The thianthrenes are furthermore destroyed under conventional reaction conditions even by the ever-present traces of water and thus lose their efficiency.
Furthermore, U.S. Pat. No. 4,289,916, EP No. 63,384 and EP No. 173,222 disclose the chlorination of toluene in the presence of Lewis acids and phenoxathiines. The o/p ratio of 0.6 obtainable according to Example 1 from EP No. 173,222 is again achieved only by the use of antimony chloride and the high amount of 0.29% by weight of co-catalyst, which are extremely unfavourable for practical application. By using FeCl.sub.3 instead of antimony chloride, an o/p ratio of 0.68 is obtained, but again only at a low reaction temperature of 5.degree. C., which is extremely unfavourable for practical application. At a reaction temperature of 50.degree. C., which is advantageous for practical application, the o/p ratio increases to 0.88 in the presence of FeCl.sub.3 and the phenoxathiine derivative claimed in EP No. 173,222, as shown by experiments which we carried out (cf. Example 27). U.S. Pat. No. 4,289,916 and EP No. 63,384 mentioned describe a favourable o/p ratio of about 0.8. In this case, too, the o/p ratio can be lowered to 0.65 by using, instead of FeCl.sub.3, antimony chloride and a reaction temperature of 20.degree. C., that is, unfavourable conditions for practical application. In addition, phenoxathiines are destroyed in the presence of traces of water.
EP No. 126,669 discloses the chlorination of toluene in the presence of Friedel-Crafts catalysts and N-substituted phenothiazines. In this case, too, the o/p ratio of 0.84 is unfavourably high.
EP No. 112,722, EP No. 154,236 and EP No. 248,931 disclose the chlorination of toluene in the presence of certain zeolites, in which an o/p ratio of about 0.3 is achieved if, for example, halocarboxylic acid halides are added as moderators. The disadvantages of this process are the substantial amounts of 5% by weight of zeolite and 1% by weight of moderators. As our own experiments have shown, this result must be paid for by the substantial disadvantage that very large amounts (up to 8% by weight) of benzyl chlorides are formed in the mixtures obtained. The formation of benzyl chlorides interferes in the subsequent conventional workup by distillation quite extensively.